1. Field of the Invention
The present invention relates to a composition containing a silicone and a structure modified pyrogenic silica, a method for producing the composition, and an article containing the composition.
2. Discussion of the Background
Silicones possess an unusual combination of properties that are retained over a wide temperature range (−100 to 250° C.). They have very good low temperature flexibility and are very stable to high temperature, oxidation, chemical and biological environments, and weathering. They possess good dielectric strength, and water repellency. Silicone resins are used in applications such as varnishes, paints, molding compounds, electrical insulation, adhesives, laminates, release coatings, sealants, caulks, adhesives, gaskets, tubing, hoses, belts, electrical insulation, encapsulating and molding applications, fabric coatings, encapsulants, and a variety of medical applications. However, many of these above-mentioned applications require improved mechanical properties of the silicone resins.
Silica fillers, such as the pyrogenic silicas described in Winnacker-Küchler Chemische Technologie, Volume 3 (1983) 4th Edition, page 77 and Ullmanns Enzyklopädie der Technischen Chemie, 4th Edition (1982), Volume 21, page 462, are traditionally added to improve the mechanical properties of a silicone, where a high degree of filling may be necessary in order to obtain increased mechanical properties. However, with a high degree of filling, it is difficult to achieve a high degree of dispersion of the fillers in the silicone. This is especially evident when continuous processes, as described in EP A 570387, are used. Additionally, the presence of the silica fillers may lead to undesirable rheological properties such as thixotropic behavior and shortened pot-life.
DE 199 43 666 A1 describes the use of pyrogenic silicas, that have been rendered hydrophobic, as fillers in silicone rubber. U.S. Pat. No. 6,331,588 also describes a silicone rubber containing pyrogenic silicas as fillers. These pyrogenic silicas are treated on the surface to render them hydrophobic to avoid the undesirable effects of the silanol groups, such as thixotropy and shortened pot-life.
As evidenced above, hydrophilic silica fillers are rendered hydrophobic and mixed with the silicone, or alternatively, hydrophilic silica fillers are rendered hydrophobic in situ while mixing with the silicone. However, in both cases, it is necessary to introduce high shear energies via the mixing units in order to achieve a low-viscosity phase. Such processes are very time-consuming and causes significant wear on the equipment. Furthermore, contamination of the silicone with other materials may also occur.
Thus, there remains a need for a silicone composition which provides the improved mechanical properties imparted by the addition of the fillers without the undesirable rheological properties.